Oceanography | Vol.33, No.3
Coral spawning is a spectacularly synchronized event. The larg-
est coral mass spawning event in the world takes place on the
Great Barrier Reef off the coast of Australia. Credit: Oren Levy
He and colleagues believe scientists
need to find out how much damage is
caused by artificial light at night and quan-
tify the benefits of less intense forms of
artificial light on corals and other sensitive
marine species. That information, accord-
ing to Wiedenmann, is vital to the future
of coastal habitats, especially coral reefs,
around the world.
LIGHT POLLUTION: A THREAT
TO GREAT BARRIER REEF AND
RED SEA CORALS
In a discovery that scientists believe will
help guide reef ecosystem protection
plans, researchers in Australia and Israel
have pinpointed artificial light at night as
a threat to coral reproduction on the Great
Barrier Reef.
Work at the University of Queensland’s
Heron Island Research Station confirms
that the Great Barrier Reef’s annual coral
spawning is dependent on an intricate mix
of conditions, with moonlight playing a
vital role. The introduction of artificial light
at night competes with moonlight and can
prevent corals from spawning, the univer-
sity’s Paulina Kaniewska, Oren Levy of Bar-
Ilan University, and their colleagues found.
The research is providing insights into
how corals have fine-tuned and coordi-
nated the release of eggs and sperm into
the water for fertilization. The results are
published in the journal eLifeSciences.
Corals have a “spread-out” nervous sys-
tem that allows them to transmit signals on
a cellular level in response to changes in
light conditions, according to Kaniewska
and coauthors. Their study suggests that
these cellular processes are triggered by
a protein similar to photosensitive melan-
opsin, one of a family of light-sensitive ret-
ina proteins, called opsins, in vertebrate
eyes. In mammals, melanopsin plays an
important role in synchronizing circadian
rhythms with the daily light-dark cycle.
The research is resolving long- standing
questions about how corals synchronize
the mass release of their reproductive
cells with the phases of the moon and
other rhythms. The effects of light on the
timing of spawning are important because
reproduction is vital to reef survival, the
scientists say. They believe that light pollu-
tion is a major threat to coral reproduction.
Coral spawning on the Great Barrier
Reef is a spectacularly synchronized
event. Changes in water temperature,
tides, sunrise and sunset, and the intensity
of moonlight trigger an annual large-scale
mass spawning of coral species.
To maximize their chances of success,
more than 130 Great Barrier Reef coral
species spawn during a time window that
is 30–60 minutes long. It’s the largest
coral mass spawning event in the world.
Most corals are “broadcast spawners”
that simultaneously release their eggs and
sperm into seawater. The egg and sperm
cells combine and develop into larvae that
settle back onto the reef to form new coral
colonies. Coral species spawn at the same
time to improve their chances of success-
ful reproduction.
How corals time this spawning behav-
ior with moon phases was an unanswered
question for decades. Then, Kaniewska
and
colleagues
exposed
Acropora
millepora— one of the dominant coral
species on the Great Barrier Reef—to
different light treatments and sampled
the corals before, during, and after their
spawning periods.
The results show that light causes
changes to gene expression and sig-
naling processes inside the corals’ cells.
The changes drive the release of egg and
sperm cells and happen only on the nights
of spawning.
Next, the researchers exposed Acropora
millepora to light conditions that mimic
artificial light at night. A mismatch in cel-
lular signaling processes prevented the
corals from spawning.
The findings also suggest that the
effects of light pollution can occur fairly
rapidly. Biologists believed that corals took
months to become tuned to the moon-
light rhythms that guide reproduction. But,
it turns out, disruption can occur within
seven days of corals’ exposure to changes
in nocturnal light.
In another study, Levy’s team recently
looked at two coral species, Acropora
eurystoma and Pocillopora damicornis,
in the Gulf of Eilat/Aqaba in the Red Sea.
“The region is undergoing urban develop-
ment that has led to severe light pollution
at night,” Levy says.
The research revealed that corals
exposed to ALAN photosynthesize less.
“Testing different lights such as blue LEDs
and white LEDs showed more extreme
impacts in comparison to yellow LEDs,”